The present invention relates to integrated microwave modules used in radio communication equipment. More particularly, the present invention deals with microwave isolation between different parts of an integrated microwave module to avoid unwanted microwave frequency bands propagation inside the integrated microwave module.
Integrated microwave modules (also referred as Monolithic Microwave Integrated Modules MMICs) include microwave circuits connected together via transmission lines, usually microstrip lines, whose architecture and length should be designed to avoid unwanted oscillations in the microwave integrated circuit.
However, at the frequencies where the integrated microwave circuit is expected to be used, especially for frequencies higher than 1 GHz, the sole appropriate design of the transmission lines is insufficient to guarantee the absence of oscillation and other techniques should be used for this purpose.
A first solution to this problem consists in filling the free space between the integrated circuits and the cover of the integrated microwave module with microwave absorber materials. For example, the covers of the integrated microwave modules are machined to create a groove and microwave absorber is stuck into the groove. The more microwave absorber used, the better is the microwave isolation between the different circuits of the integrated microwave module. However, usual microwave absorbers are narrow band materials that eliminate only a narrow frequency band while the unwanted microwave frequency bands which perturb the behavior of the integrated microwave modules extend beyond the frequency band eliminated by the microwave absorbers. Moreover, the use of good quality microwave absorbers raises significantly the cost of integrated microwave modules.
Another solution is disclosed in U.S. Pat. No. 6,121,833 (assigned to the assignee of the present application) in the case of a high gain microwave amplifier. For avoiding the phenomenon of interfering re-injections between the output and the input of the amplifier, waveguides are provided for in the spaces situated below the cover of the MMIC. The cutoff frequencies of these waveguides is chosen higher than the upper limit of the working passband to be amplified. The free space below the cover is subdivided into waveguides by including, in this space, metal bars constituting the metal walls of the waveguides.
The design of metal bars in the free space requires very high precision tools and increase also the price of the integrated microwave modules.
A particular object of the present invention is to provide an integrated microwave module which comprises efficient microwave isolation means for a lower cost and not requiring additional metal bars to be included below the cover of the integrated microwave module.
Another object of the invention is to provide a method for manufacturing such integrated microwave modules.